In a wireless communication system, a variety of errors are inherent to radio signal propagation over the air, thus making data transmission difficult. Because a radio channel experiences multi-path fading, path loss that becomes serious as a User Equipment (UE) is farther from a Base Station (BS), etc. as well as thermal noise modeled as Additive White Gaussian Noise (AWGN), it is more difficult to transmit a signal reliably.
To ensure transmission reliability against various changes in channel status and errors during wireless communications, techniques such as 1) Forward Error Correction (FEC) or channel coding, and 2) Automatic Repeat reQuest (ARQ) or Hybrid Automatic Repeat reQuest (HARQ) are widely used.
Among them, HARQ will be described in brief.
FIG. 1 illustrates a signal flow referred to for describing the principle of HARQ.
Referring to FIG. 1, HARQ is a hybrid technology of conventional schemes in combination, that is, ARQ in the Medium Access Control (MAC) layer and channel coding in the physical layer. In HARQ, an erroneous initial transmission packet P1A is stored until a retransmission signal P1B is received because the initial transmission packet P1A retains information to a certain extent despite errors, and decoded after being soft-combined with the retransmission signal P1B or along with the retransmission signal P1B without soft combining.
The initial transmission signal P1A and the retransmission signal P1B are the same or different transmission packets created from the same channel encoder input P1. A packet P2A is produced from a new channel encoder input P2.
There are two types of HARQ schemes: Chase Combining (CC) and Incremental Redundancy (IR).
In CC HARQ, a retransmission packet is identical to previously transmitted packets with reception errors and soft-combined with them. This scheme is also called soft-combining HARQ.
FIG. 2 conceptually illustrates an exemplary packet transmission in CC HARQ.
Referring to FIG. 2, an original packet and a retransmission packet based on CC are shown. The original transmission signal that was turbo-encoded at a coding rate of 1/3 and punctured has 16 bits as illustrated in the left middle part of FIG. 2. If a transmitter receives a Negative ACKnowledgment (NACK) for the original transmission packet from a receiver, it retransmits a perfectly identical packet as illustrated in the right middle part of FIG. 2.
Since the same packets are transmitted at different time points, time diversity is effected and Signal-to-Noise Ratio (SNR) increases each time packets are combined, even though errors are generated in the CC HARQ scheme. Therefore, an error-free reception probability is increased.
FIG. 3 illustrates the concept of packet transmission in IR HARQ scheme.
Referring to FIG. 3, a retransmission packet has a different structure from a previous transmission packet with errors in IR scheme. As its appellation implies, the IR scheme increments redundancy at each transmission.
The IR HARQ scheme increases a channel coding gain or a repetition rate during rate matching, when a retransmission packet for an erroneous packet is transmitted. The resulting increase in valid channel coding gain adaptively increases the robustness of the retransmission packet against noise during retransmission. Because redundancy bits are increased in number and information bits are deceased in number in each retransmission packet in IR, an IR HARQ buffer is additionally used during the rate matching to handle a change in transmission rate during retransmission. A punctured packet illustrated in the left middle part of FIG. 3 is an original transmission packet.
In comparison between final decoder input signals illustrated in FIGS. 2 and 3, the CC decoder input contains information in a total of 16 bits, each bit having an increased SNR by soft combining, whereas the IR decoder input contains information in more bits than the CC decoder input, even though bit combining is not performed. Thus it can be concluded that IR HARQ usually outperforms CC HARQ.
These HARQ schemes increase reception SNR gradually because redundancy information increases at each retransmission by soft combining or IR. Even though the reception SNRs of individual transmissions may not meet a minimum requirement, a combination of a plurality of packets transmitted at repeated retransmissions has a final SNR satisfying the requirement. Accordingly, successful packet transmission is possible.
However, transmission of retransmission packets for a long time leads to an increased latency. In case of severe interference from neighbor cells, it often occurs that CC does not ensure its effectiveness.